1. Field of the Invention
The present invention relates to methods of preparing grinding media consisting essentially of sintered titanium dioxide (TiO.sub.2) particles and to the resulting grinding media produced by such methods. The invention further encompasses methods of using such grinding media in media mills for reducing the particle size of particulate material, particularly titanium dioxide pigments.
2. Description of the Related Art
Media-milling is an established technology for reducing the particle size of particulate material such as titanium dioxide (TiO.sub.2), and media-milling equipment is commercially available. Typically, when TiO.sub.2 particles are media-milled, a slurry of TiO.sub.2 particles is fed into a grinding chamber charged with grinding media. The grinding media are stirred in the grinding chamber by a series of discs attached to a rotating shaft. The motion of the stirring discs and grinding media is perpendicular to the direction in which the slurry is pumped; thus, the TiO.sub.2 particles are sheared by the media. A screen is often used to keep the grinding media inside the grinding chamber, while allowing the TiO.sub.2 slurry to flow out of the mill. Media-milling may be performed with various types and shapes of grinding media such as beads, sand, or pebbles. Other examples of grinding media include particles of zirconium silicate, alumina, silicon carbide, silica, and zirconia. Often, the media-milling process is adjusted to provide a final product having the desired particle size, optics, durability, gloss, and dispersibility.
As discussed above, different types of grinding media are known. For example, Japanese Kokai Patent Application No. Sho 64 [1989]-83548 discloses a process to mix and crush at least two ceramic powders containing a titanium element (e.g., titania, titania plumbate) and a zirconium element (e.g., zirconia, zirconia plumbate) using precious stones composed of titanium dioxide and zirconium dioxide as media. Recognizing that abrasion of the media will occur, it is disclosed that using both titanium- and zirconium-containing media with ceramic powders containing a titanium element and a zirconium element reduces the variation in the ratio of Ti:Zr components in the ceramic powders which occurs from before and after crushing of the ceramic powders.
It is understood that commercially available grinding media from Glen Mills, Inc. (Clifton, N.J. 07012) is prepared by physically blending titania (TiO.sub.2), alumina (Al.sub.2 O.sub.3), and silica (SiO.sub.2) particles. The final grinding media composition comprises 77.7 wt. % TiO.sub.2, 17.4 wt. % Al.sub.2 O.sub.3, and 4.6 wt. % SiO.sub.2. The grinding media have a specific gravity of 3.9 g/cc, hardness of 1100 kgf/mm.sup.2, compressive strength of 28 kgf/mm.sup.2, and a packed density of 2.4 kg/1. The particle size of the media is described as being in the range of 0.8 mm-5.0 mm.
Also, calcined TiO.sub.2 can be used as granular scrubs for removing TiO.sub.2 deposits from the internal surfaces of cooling conduits. Rick, U.S. Pat. No. 2,721,626 discloses adding relatively dense, hard abrasive particles such as calcined, agglomerated titanium dioxide, silica, or sand into a cooling conduit containing a hot suspension of TiO.sub.2 particulate material. It should be recognized that calcined TiO.sub.2 scrubs are typically prepared by heating the TiO.sub.2 particles to a maximum temperature of 1000.degree. C., and the surface area of most of the TiO.sub.2 particles is decreased by this heating step. The calcined TiO.sub.2 scrubs are coarse and have a relatively large particle size in the range of 4 to 20 mesh (4750 to 850 microns) and are typically porous and fragile.
As discussed above, it is recognized that certain grinding media comprising TiO.sub.2 particles are commercially available from Glen Mills, Inc. However, the process used to produce such grinding media is cost-intensive, since it involves making spherical particles from a physical mixture of titania, alumina, and silica. Further, these grinding media have a relatively high weight percentage of alumina which contributes to their having generally good wear-resistance properties.
One object of the present invention is to provide new methods for preparing grinding media consisting essentially of TiO.sub.2 particles. Another object of this invention is to provide grinding media having good wear-resistance which contain a relatively low weight percentage of alumina. The present invention provides such methods and products prepared by these methods, along with methods for using such grinding media in media-milling operations.